Method for producing silicon

ABSTRACT

A silicon production process which improves the production efficiency of trichlorosilane while an industrially advantageous output is ensured and the amount of the by-produced tetrachlorosilane is suppressed. This process does not require a bulky reduction apparatus for the by-produced tetrachlorosilane, can construct a closed system, which is a self-supporting silicon production process, can easily control the amount of the by-produced tetrachlorosilane and therefore can adjust the amount of tetrachlorosilane to be supplied to a tetrachlorosilane treating system when the tetrachlorosilane treating system is used. 
     This process comprises a silicon deposition step for forming silicon by reacting trichlorosilane with hydrogen at a temperature of 1,300° C. or higher, a trichlorosilane forming step for forming trichlorosilane by contacting the exhausted gas in the above silicon deposition step to raw material silicon to react hydrogen chloride contained in the exhausted gas with silicon, and a trichlorosilane first recycling step for separating trichlorosilane from the exhausted gas in the trichlorosilane forming step and recycling it to the silicon deposition step.

This application is the national phase under 35 U.S.C. §371 of PCTInternational Application No. PCT/JP02/10848 which has an Internationalfiling date of Oct. 18, 2002, which designated the United States ofAmerica.

FIELD OF THE INVENTION

The present invention relates to a novel silicon production process.More specifically, it relates to a process of producing silicon througha reaction between trichlorosilane (may be abbreviated as TCShereinafter) and hydrogen, which is capable of carrying out thetreatment of tetrachlorosilane (may be abbreviated as STC hereinafter)formed by a silicon deposition reaction industrially extremelyadvantageously.

DESCRIPTION OF THE PRIOR ART

High-purity silicon obtained from TCS can be produced from a reactionbetween TCS and hydrogen. As an industrial production process, there isknown a so-called “Siemens process” in which the surface of a siliconrod is heated and TCS is supplied to the rod together with hydrogen todeposit silicon on the rod in order to obtain a grown polycrystalsilicon rod.

The above deposition reaction is generally carried out at a temperatureof 900 to 1,250° C., substantially 900 to 1,150° C. to deposit siliconstably, and STC and hydrogen chloride are by-produced by the depositionreaction.

As for the amounts of STC and hydrogen chloride formed by the silicondeposition reaction at the above temperature range employed by the aboveSiemens process, STC is formed in an overwhelming amount.

When 1 ton of high-purity silicon is to be produced by the Siemensprocess at the above temperature range, STC is formed in an amount of 15to 25 tons and hydrogen chloride is formed in an amount of 0.1 to 1tons.

STC by-produced by the silicon deposition reaction is a compound whichis chemically much more stable than TCS. As shown in the followingreaction formulas, as STC is contained in TCS more, the rate of thesilicon deposition reaction becomes lower, thereby greatly reducing theefficiency of the silicon deposition reaction due to an equilibriumimpediment factor as well.

-   Delay of Deposition Reaction Rate    TCS+H₂→Si+HCl+DCS+TCS+STC  (1)    STC+H₂→Si+HCl+DCS+TCS+STC  (2)    (DSC is dichlorosilane)

The yield of Si under the same reaction conditions is the formula(1):formula (2)=5:1.

-   Equilibrium Impediment    TCS+H₂→Si+HCl+DCS+TCS+STC    In the above formula, when STC in a formed system is existent in a    raw material system, equilibrium tends to be left-sided.    (law of Le Chatelier)

Therefore, in a system for carrying out the silicon deposition reactionon an industrial scale, part or all of STC formed in large quantities bythe silicon deposition reaction must be discharged to a treating system(to be referred to as “STC treating system” hereinafter) nearby or at adistance.

Examples of the STC treating system include a system for producing fumedsilica or quartz by hydrolyzing STC with oxyhydrogen flames and anepitaxial system for silicon wafers.

However, the consumption of STC in the STC treating system is affectedby demand for fumed silica or the like produced therefrom. When thedemand decreases, surplus STC which cannot be treated must be abandoned.Thus it is difficult to balance between the production of silicon anddemand for STC, and a basic solution to the treatment of STC which isformed in large quantities has not yet been found.

To cope with the above problem, closed systems have been proposed byJP-A 52-133022 and JP-A 10-287413 (the term “JP-A” as used herein meansan “unexamined published Japanese patent application”) as aself-supporting process for reducing the output of STC and dischargingno STC. However, these systems are only based on an ideal system. Thatis, the system of JP-A 52-133022 provides a closed system technology forspecifying the composition of a gas used for the deposition of siliconto deposit silicon at a temperature of 900 to 1,250° C. in order tosuppress the by-production of STC. However, as seen in its Examples, asilicon deposition reaction system is placed under conditions close toan equilibrium state (ideal system) by making the amount of the suppliedgas extremely small for the reaction area. It is difficult to ensure anindustrially effective output under the above conditions. When thesupply of the raw material gas having the above composition is increasedto ensure the output, the reaction rate of TCS greatly lowers, whichmakes it difficult to carry out the silicon deposition reaction on anindustrial scale.

In order to produce silicon in an industrially advantageous amount atthe above deposition temperature, the proportion of gaseous hydrogenmust be reduced to improve the reaction rate of TCS, thereby sharplyincreasing the by-production of STC.

Therefore, to carry out the deposition of silicon at a relatively lowtemperature of 1,250° C. or lower on an industrial scale, theby-production of a large amount of STC is inevitable as described aboveand a technology for carrying out a closed system industrially has notyet been completed.

As a process for producing TCS from STC, JP-A 57-156318 proposes aprocess for obtaining TCS by converting STC into TCS through hydrogenreduction and then reacting the hydrogen chloride of the reaction gaswith low-purity silicon of a metallurgical grade (metallurgical-gradesilicon). However, even this process does not provide a solution to theproblem encountered in the step in which STC is formed in anoverwhelming amount.

Meanwhile, a process for carrying out a silicon deposition reaction ataround 1,410° C. which is the melting point of silicon is proposed byJP-A 11-314996. However, researches are not made into the gascomposition at the above deposition temperature as well as an industrialprocess.

OBJECTS OF THE INVENTION

It is therefore a first object of the present invention to provide asilicon production process which improves the production efficiency ofTCS while an industrially advantageous output is ensured and the amountof by-produced STC is reduced.

It is a second object of the present invention to provide aself-supporting silicon production process which does not require abulky reduction apparatus for by-produced STC and enables theconstruction of a closed system.

It is a third object of the present invention to provide a siliconproduction process which can easily control the amount of theby-produced STC and therefore can adjust the amount of STC to besupplied to an STC treating system to any value when the STC treatingsystem is installed.

Other objects and advantages of the present invention will becomeapparent from the following description.

SUMMARY OF THE INVENTION

The inventors of the present invention have conducted intensive studiesto attain the above objects and have found that an industrial processwhich can reduce the amount of the formed STC to an extremely smallvalue unattainable by the Siemens process by carrying out a silicondeposition reaction between TCS and hydrogen at a specific hightemperature range which has not been used for industrial production canbe established. Thus, the present invention has been accomplished basedon this finding.

That is, the first object and advantage of the present invention areattained by a silicon production process comprising a silicon depositionstep for forming silicon by reacting trichlorosilane with hydrogen at atemperature of 1,300° C. or higher, a trichlorosilane forming step forforming trichlorosilane by contacting the exhausted gas in the abovesilicon deposition step to raw material silicon to react hydrogenchloride contained in the exhausted gas with silicon, and atrichlorosilane first recycling step for separating trichlorosilane fromthe exhausted gas in the trichlorosilane forming step and recycling itto the silicon deposition step.

They have found that a closed system for discharging substantially noSTC to the outside of the process by reducing STC with hydrogen can beconstructed because the amount of the by-produced STC is extremelysmall.

That is, the above second object and advantage of the present inventionare advantageously attained by a silicon production process comprising asilicon deposition step for forming silicon by reacting trichlorosilanewith hydrogen in a hydrogen/trichlorosilane molar ratio of 10 or more ata temperature of 1,300° C. or higher, a trichlorosilane forming step forforming trichlorosilane by contacting the exhausted gas in the abovesilicon deposition step to raw material silicon to react hydrogenchloride contained in the exhausted gas with silicon, a trichlorosilanefirst recycling step for separating trichlorosilane from the exhaustedgas in the trichlorosilane forming step and recycling it to the silicondeposition step, a tetrachlorosilane reducing step for reducingtetrachlorosilane contained in the residue after the separation oftrichlorosilane in the trichlorosilane first recycling step withhydrogen to obtain trichlorosilane, and a trichlorosilane secondrecycling step for recycling the exhausted gas in the tetrachlorosilanereducing step to the above trichlorosilane forming step.

Further, the inventors of the present invention have found that theamount of the by-produced STC can be adjusted to an extremely wide rangewithout affecting the quality of the obtained silicon by changing thehydrogen/TCS molar ratio in the silicon deposition reaction at the abovehigh temperature range and the amount of STC to be supplied to an STCtreating system can be thereby easily controlled even when the STCtreating system is installed.

Therefore, the above third object and advantage of the present inventionare attained by a silicon production process comprising a silicondeposition step for forming silicon by reacting trichlorosilane withhydrogen at a temperature of 1,300° C. or higher, a trichlorosilaneforming step for forming trichlorosilane by contacting the exhausted gasin the above silicon deposition step to raw material silicon to reacthydrogen chloride contained in the exhausted gas with silicon, atrichlorosilane first recycling step for separating trichlorosilane fromthe exhausted gas in the trichlorosilane forming step and recycling itto the silicon deposition step, a tetrachlorosilane reducing step forreducing a part of tetrachlorosilane contained in the residue after theseparation of trichlorosilane in the trichlorosilane first recyclingstep with hydrogen to obtain trichlorosilane, a trichlorosilane secondrecycling step for recycling the exhausted gas in the tetrachlorosilanereducing step to the above trichlorosilane forming step, and atetrachlorosilane supply step for supplying the balance oftetrachlorosilane supplied to the tetrachlorosilane reducing step to atetrachlorosilane treating system, wherein the amount oftetrachlorosilane to be supplied to the above tetrachlorosilane treatingsystem is changed in the above tetrachlorosilane supply step by changingthe molar ratio of hydrogen to trichlorosilane to be supplied to thesilicon deposition step.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 show silicon deposition apparatuses suitably used forcarrying out the process of the present invention.

FIG. 3 is a graph showing the tendencies of the amount of STC and theamount of hydrogen chloride at each deposition temperature.

FIG. 4 is a process diagram showing a typical embodiment of the presentinvention.

FIG. 5 is a process diagram showing another typical embodiment of thepresent invention.

PREFERRED EMBODIMENTS OF THE INVENTION

In the present invention, the silicon deposition step is to react TCSwith hydrogen at a temperature of 1,300° C. or higher, preferably 1,300to 1,700° C., more preferably the melting point of silicon or higher and1,700° C. or lower.

In the above silicon deposition step, the method of carrying out thedeposition of silicon industrially continuously is not particularlylimited. The method preferably uses the apparatus shown in FIG. 1, whichis the apparatus basically comprises (1) a cylindrical vessel 1 havingan opening 2 which is a silicon output port at the lower end, (2) aheater 3 which can heat the inner wall of the above cylindrical vessel 1from its lower end to any height at a temperature equal to or higherthan the melting point of silicon, (3) a chlorosilane feed pipe 4 forsupplying a chlorosilane A, which is open to a space 5 surrounded by theinner wall heated at a temperature of 1,300° C. or higher of the abovecylindrical vessel 1 and faces downward, (4) a seal gas feed pipe 6 forsupplying a hydrogen gas as a seal gas B into a space formed by theinner wall of the cylindrical vessel 1 and the outer wall of thechlorosilane feed pipe 4, and (5) a cooling material 9 mounted below theabove cylindrical vessel 1 with a space therebetween.

In order to carry out the collection of an exhaust gas D from thecylindrical vessel 1 efficiently in the above apparatus, the cylindricalvessel 1 and the cooling material 9 are preferably covered with a closedvessel 7 provided with an output pipe 12 for the exhaust gas D.

A seal gas C such as nitrogen, hydrogen or argon is preferably suppliedby a seal gas feed pipe 11 into a space formed by the outer wall of thecylindrical vessel 1 and the inner wall of the closed vessel 7.

In the above apparatus, the heater 3 for heating the cylindrical vessel1 is preferably a high-frequency coil. The cylindrical vessel 1 ispreferably made from a material which can be heated with high frequencywaves and has durability at the melting point of silicon. In general,carbon is preferably used. Carbon coated with silicon carbide, thermallydecomposed carbon or boron nitride is preferred because it can improvethe durability of the cylindrical vessel and the purity of a siliconproduct.

In the above apparatus, a chlorosilane supplied from the chlorosilanefeed pipe 4 may be mixed with hydrogen. A chlorosilane gas or a mixedgas of a chlorosilane and hydrogen is supplied into the space 5 of thecylindrical vessel 1 together with hydrogen as a seal gas supplied bythe seal gas feed pipe 6 and heated by the heater 3 to deposit siliconon the inner wall of the cylindrical vessel 1.

When the cylindrical vessel 1 is heated at a temperature of the meltingpoint of silicon or higher, the deposited silicon flows down over theinner wall of the cylindrical vessel as a silicon molten liquid and isdropped from the opening 2 as a droplet 14 spontaneously. Therefore, theinside of the cylindrical vessel can be always kept in a fixed statewithout carrying out the periodical operation of increasing thetemperature.

When the cylindrical vessel 1 is heated at a temperature of 1,300° C. orhigher and lower than the melting point of silicon, silicon separatesout as a solid. In this case, when the amount of the deposited siliconreaches a certain value, the heat output is increased or the supply ofthe gas is reduced to raise the temperature of the cylindrical vessel 1to a temperature equal to or higher than the melting point of silicon inorder to melt part or all of the deposit and drop it. Thus silicon canbe collected and deposition can be carried out continuously.

In this text, as a reaction between TCS and hydrogen occurs on thedeposition surface of the above cylindrical vessel, the reactiontemperature is the heating temperature of the cylindrical vessel.

When the cylindrical vessel 1 is heated at a temperature around themelting point of silicon, silicon may separate out partly in a solidstate and partly in a molten state. When the amount of the depositedsilicon reaches a certain value, the temperature is raised to melt partor all of the solid and drop it, thereby collecting it as in the abovemethod.

A droplet of the silicon molten liquid or partly molten solid siliconfalling from the above cylindrical vessel is dropped on the coolingmaterial 9 which is a receptacle to be solidified and collected assilicon 8.

When silicon is to be dropped as a molten liquid, before falling siliconis received by the cooling material 9 or while silicon is falling beforeit is received by the cooling material 9, the silicon molten liquid maybe made fine by a known method.

The solidified silicon deposit dropped on the cooling material 9 can betaken out from the closed vessel 7 after the deposition reaction isstopped, preferably taken out while the deposition reaction iscontinued. To collect silicon while the reaction is continued, there isemployed a method in which the temperature of the cylindrical vessel 1is adjusted to 1,300° C. or higher and lower than the melting point toprevent the silicon molten liquid from falling and a valve installedbetween the deposition reactor and a collection vessel is closed to opena collection unit or a method in which a grinder installed in acollection unit is used to apply mechanical force to the silicon depositsolidified in the collection unit to grind it to a certain extent andsilicon E is taken out from a silicon output port 13 located below thecooling material 9 intermittently.

In the reactor shown in FIG. 1, the raw material gas is supplied intothe inside of the cylindrical vessel. As shown in FIG. 2, a reactor inwhich the cylindrical vessel 1 has a multi-structure having an openingat the bottom and the raw material gas A is supplied into a space 15formed between cylinders from above may be preferably used.

When the apparatus shown in FIG. 2 is used, in order to prevent theblockage of the above space by the deposition of silicon, it isrecommended to set the reaction temperature to the melting point ofsilicon or higher.

Heating means 16 such as a high-frequency generating coil or an electricheater is placed in the central space of the multi-cylindrical vessel toheat an inner cylindrical vessel in particular to the full. In thiscase, an inert gas is preferably existent in the closed space forinstalling the heating means 16. The closed space may be evacuated. Aheat insulator (unshown) for protecting the heating means 16 may befurther provided.

In the process of the present invention, the reaction temperature mustbe 1,300° C. or higher. This is because the amount of STC formed in thesilicon deposition step is effectively reduced and the amount ofhydrogen chloride for facilitating the formation of TCS from STC isincreased.

FIG. 3 shows the trends of the amount of the by-produced STC and theamount of the by-produced hydrogen chloride at 1,050° C., 1,150° C.,1,350° C. and 1,410° C. in the reaction when the molar ratio of hydrogento TCS is 10. As understood from FIG. 3, when the temperature is higherthan about 1,300° C., the amount of the by-produced STC greatlydecreases and the amount of the by-produced hydrogen chloride increases.

Although the reason why the reaction result obtained when the depositionreaction temperature is 1,300° C. or higher and the reaction resultobtained when the conventionally proposed deposition reactiontemperature is 1,250° C. or lower differ from each other as describedabove is not elucidated yet, it is assumed that the temperature of theboundary film near the deposition surface is closely connected withthis.

That is, the supplied TCS is fully activated in the high-temperatureboundary film to increase its conversion into silicon whereas theactivation of TCS is rather insufficient in the low-temperature boundaryfilm with the result that a reaction for disproportionating twomolecules of TCS into dichlorosilane and STC readily occurs and afurther reaction does not take place. In fact, in the depositionreaction which is carried out at the temperature of the presentinvention, the amount of the formed dichlorosilane is much smaller thanin the Siemens process of the prior art.

It has been found through studies on the present invention that thedeposition reaction temperature which can attain the object of thepresent invention fully is 1,300° C. or higher. By carrying out thesilicon deposition reaction at the above temperature, the amount of theformed STC can be reduced to ½ or less that of the Siemens process ofthe prior art in some cases and to ⅓ in others. At the same time, theamount of the formed hydrogen chloride can be increased to 5 times ormore that of the Siemens process in some cases and 10 times in others.Moreover, the silicon deposition rate can be increased to 5 times ormore that of the Siemens process in some cases and to 10 times inothers, and the reaction rate of the raw material TCS can be increasedto 1.5 times or more that of the Siemens process in some cases and to 2times in others, thereby making it possible to produce a large amount ofsilicon with a very small-sized deposition reactor.

It is preferred to adjust the molar ratio (H₂/TCS) of hydrogen to TCS tobe used in the above reaction to 10 or more, preferably 15 to 30 so asto effectively reduce the amount of the formed STC and greatly increasethe amount of the formed hydrogen chloride in the silicon depositionstep.

Further, the pressure of the above reaction is not particularly limitedbut preferably normal pressure or higher.

In the present invention, the TCS forming step is the step of formingTCS by contacting the exhausted gas in the silicon deposition step toraw material silicon to react hydrogen chloride contained in the gaswith silicon.

The exhausted gas in the silicon deposition step contains hydrogenchloride and STC as the main products and small amounts ofdichlorosilane (to be abbreviated as DCS hereinafter) and oligomers ofchlorosilanes. The gas also contains unreacted TCS. When this mixed gasis contacted to raw material silicon, hydrogen chloride selectivelyreacts to form TCS. This reaction is an exothermic reaction which canproduce TCS energetically much more advantageously than an endothermicreaction for producing TCS by reducing STC with hydrogen.

As the above raw material silicon may be used known metallurgical-gradesilicon which is generally used as a raw material for producing siliconwithout restriction.

Any reactor capable of contacting the raw material silicon to theexhausted gas from the deposition reaction may be used as the reactorused for the above reaction. For example, a fluidized bed reactor forreacting a raw material silicon powder with a gas while it is fluidizedby the gas is preferred for industrial-scale production. As means ofadjusting the temperature of the above fluidized bed reactor may be usedany known method. For example, a heat exchanger is installed internal orexternal to the fluidized bed, or the temperature of a preheating gas isadjusted.

In the trichlorosilane forming step, the temperature for starting areaction between silicon and hydrogen chloride is almost 250° C.Therefore, the reaction temperature must be 250° C. or higher. Toimprove the yield of TCS, it is preferably 400° C. or lower. In order tocontinue the reaction industrially stably, the reaction temperature isparticularly preferably adjusted to 280 to 350° C.

In the present invention, the exhausted gas obtained from the TCSforming step is supplied to the TCS first recycling step to separate TCScontained in the gas and recycle it to the above silicon depositionstep. Hydrogen to be recycled is preferably obtained by removing partsof chlorosilanes from the gas. Various known methods may be used toseparate hydrogen from the chlorosilanes but this can be easily carriedout by cooling the gas industrially. To cool the gas, it may just passthrough a cooled heat exchanger or cooled with a condensed and cooledcondensate. These methods may be used alone or in combination. The abovecooling temperature is not limited if parts of chlorosilanes arecondensed but preferably 10° C. or lower, more preferably −10° C. orlower, the most preferably −30° C. or lower to improve the purity ofhydrogen. Most impurities contained in the raw material silicon, such asa heavy metal, phosphorus and boron, can be removed from hydrogen by theoperation of removing parts of chlorosilanes, and the purity of thedeposited silicon can be improved.

Parts of chlorosilanes are separated and the collected hydrogen hassufficiently high purity but it may contain a relatively large amount ofa boron compound according to separation conditions. Therefore,according to the required purity of a silicon product, it is desiredthat the boron compound should be removed from the hydrogen gas. Themethod of removing the boron compound is not particularly limited but amethod in which a substance having a functional group such as —NR₂ (R isan alkyl group having 1 to 10 carbon atoms), —SO₃H, —COOH or —OH iscontacted to the above hydrogen gas is preferred. The simplest method isto contact an ion exchange resin having any one of the above functionalgroups to the hydrogen gas.

Any known method may be employed to separate TCS from the gas formedafter the production of TCS. For instance, when the above hydrogen isseparated, TCS can be separated by purifying the condensed gas bydistillation. The residue after the separation of TCS by distillationpurification includes a small amount of DCS as a light end, and STC,small amounts of chlorosilanes, oligomers of chlorosilanes and heavymetal compounds as a heavy end.

The above light end does not need to be separated from TCS. However,when it is separated, it is supplied to the STC reduction reactiontogether with STC or gasified to be supplied to the TCS forming stepagain. Since the heavy end contains STC as the main component, after STCand heavy metal compounds are separated by a known method, STC isconverted into TCS by the reduction step to be described hereinafter ortreated in another treating step for its effective use.

To further remove the boron compound from the collected chlorosilanes asa liquid according to the required purity of a silicon product, afterthe above solid or liquid compound having a functional group iscontacted to the chlorosilanes, the reaction product can be purified bydistillation as required.

The residue after the separation and collection of most of STC from theheavy end is generally neutralized and abandoned. In this case, as forchlorine lost by this, hydrogen chloride or a chlorosilane may besupplied into the system to compensate for the loss.

It is understood that the closed system in the present inventionincludes a mode that hydrogen which is inevitably reduced in quantity issupplied.

In the present invention, drive force for recycling is required torecycle the gas. Any known gas pressure device for generating driveforce may be employed. As for the installation position of the gaspressure device, it may be installed at an upstream of the TCS formingstep in which the apparatuses for the TCS forming step and the step ofseparating hydrogen from chlorosilanes can be reduced in size,preferably at an upstream of the silicon deposition step in which thetotal amount of substances for causing a trouble in the pressure deviceis the smallest.

The silicon production process of the present invention can reduce theamount of STC formed by the silicon deposition reaction to ¼ that of theSiemens process of the prior art in some cases and to ⅕ in others by amultiplication effect obtained by carrying out the silicon depositionreaction at 1,300° C. or higher and adjusting the molar ratio ofhydrogen to TCS in the deposition reaction.

Therefore, the process for producing silicon with a closed system forconverting the entire amount of STC formed in the silicon depositionstep into TCS to be recycled can be extremely advantageously realized.In addition, the size of the apparatus for recycling the gas can bereduced to about ½ or less that of the Siemens process owing to the highreaction rate of TCS and the high yield of silicon.

FIG. 4 is a process diagram showing the process for producing siliconwith the above closed system. As shown in the figure, the processcomprises a silicon deposition step 101 for forming silicon by reactingTCS with hydrogen in a hydrogen/TCS molar ratio of 10 or more at atemperature of 1,300° C. or higher, a TCS forming step 102 for formingTCS by contacting the exhausted gas in the above silicon deposition stepto raw material silicon to react hydrogen chloride contained in the gaswith silicon, a TCS first recycling step 103 for separating TCS from theexhausted gas in the TCS forming step and recycling it to the silicondeposition step, an STC reducing step 104 for obtaining TCS by reducingSTC contained in the residue after the separation of TCS in the TCSfirst recycling step with hydrogen, and a TCS second recycling step 105for recycling the exhausted gas in the STC reducing step to the aboveTCS forming step.

A hydrogen/trichlorosilane separation step 201 for separating hydrogenfrom chlorosilans by condensation is preferably carried out after theabove TCS forming step 102 as described above. In the TCS firstrecycling step 103, the separation of TCS is carried out by adistillation purifying step 202 for purifying a condensate solution fromthe above hydrogen/trichlorosilane separation step 201 by distillation.

In the above mode, the STC reducing step 104 is a step for convertingSTC contained in the residue after the separation of TCS into TCS byreacting it with hydrogen after the separation of hydrogen as required.As the reaction conditions may be used known conditions withoutrestriction. To improve the conversion rate and amount of STC into TCS,the reduction reaction temperature is adjusted to 1,300° C. or higher,preferably 1,300 to 1,700° C., particularly preferably 1,410 to 1,700°C. When the reduction reaction temperature is lower than 1,410° C., thatis, lower than the melting point of silicon, the deposition of solidsilicon in the inside of the reactor can be suppressed by adjusting themolar ratio of hydrogen to the supplied STC to 10 or less. When thereduction reaction temperature is 1,410° C. or higher, the deposit isdischarged to the outside of the system as a molten liquid even underconditions that silicon separates out, whereby the molar ratio ofhydrogen to STC can be adjusted without restriction.

The reactor used for this reaction is not limited to a particularstructure if it can attain a reaction temperature condition. Theapparatus shown in FIG. 1 or FIG. 2 used for the silicon depositionreaction is preferably used as a reduction reactor. That is, in thiscase, STC is supplied from the chlorosilane feed pipe 4.

In the present invention, the above TCS second recycling step 105 is astep for recycling the exhausted gas in the STC reduction step 104 tothe above TCS forming step 102 to react hydrogen chloride contained inthe gas with raw material silicon. The inventors of the presentinvention have found that the composition of an exhausted gas from thesilicon deposition reaction is very similar to the composition of anexhausted gas from the STC reduction reaction under conditions shown inthe present invention. That is, in the TCS forming step 102, theexhausted gases may be treated using a plurality of reactors or may betreated together by using a single reactor.

The silicon production process of the present invention has an advantageobtained by carrying out the deposition of silicon at a temperature of1,300° C. or higher and an advantage that the amount of the formed STCcan be adjusted to a wide range from the above mentioned extremely smallamount to the same amount as that of the Siemens process of the priorart by changing the molar ratio of hydrogen to TCS in the silicondeposition step without changing the quality of silicon obtained by thesilicon deposition step.

That is, in the Siemens process of the prior art, the hydrogen/TCS molarratio is controlled under fixed condition at a range of 5 to 10. It isknown that when the molar ratio changes during deposition for somereason, the shape and the surface state of the deposit deteriorateextremely, thereby reducing the value of a product, and that a sharptemperature distribution is partially formed during deposition with theresult that the deposit is blown, thereby making it difficult tocontinue deposition any more. Therefore, the operation of greatlychanging the molar ratio is actually impossible industrially.

In contrast to this, since the deposition of silicon is carried out at ahigh temperature close to a melting temperature of 1,300° C. in thesilicon production process of the present invention, a depositionreaction proceeds while silicon is partially molten or totally molten ata temperature of its melting point or higher. Silicon can be collectedby melting part or all of the deposit from a heated body.

Therefore, even when part of the deposition surface is molten by achange in the above molar ratio, it is substantially unnecessary to takeinto consideration the shape and surface state of the deposit and it istherefore possible to adjust the hydrogen/TCS molar ratio to any valuefrom any point of time.

That is, according to another embodiment of the present invention, asshown in the process diagram of FIG. 5, there is provided a siliconproduction process comprising a silicon deposition step 101 for formingsilicon by reacting trichlorosilane with hydrogen at a temperature of1,300° C. or higher, a trichlorosilane forming step 102 for formingtrichlorosilane by contacting the exhausted gas in the above silicondeposition step to raw material silicon to react hydrogen chloridecontained in the gas with silicon, a trichlorosilane first recyclingstep 103 for separating trichlorosilane from the exhausted gas in thetrichlorosilane forming step and recycling it to the silicon depositionstep, a tetrachlorosilane reducing step 104 for obtainingtrichlorosilane by reducing part of tetrachlorosilane contained in theresidue after the separation of trichlorosilane in the trichlorosilanefirst recycling step with hydrogen, a trichlorosilane second recyclingstep 105 for recycling the exhausted gas in the tetrachlorosilanereducing step to the above trichlorosilane forming step, and atetrachlorosilane supply step 106 for supplying the balance oftetrachlorosilane supplied to the tetrachlorosilane reducing step to atetrachlorosilane treating system, and the process in which the amountof tetrachlorosilane to be supplied to the tetrachlorosilane treatingsystem is changed by altering the molar ratio of hydrogen totrichlorosilane to be supplied to the silicon deposition step.

In the silicon production process of the present invention, when themolar ratio of hydrogen to TCS is made small, the amount of the formedSTC can be made large and when the molar ratio is made large, the amountof the formed STC can be made small. Therefore, the deposition ofsilicon may be carried out by controlling the molar ratio according to arequired amount of STC in the STC treating system 203 in the STC supplystep. In general, the molar ratio of hydrogen to TCS can be controlledto a range of 5 to 30.

The above STC treating system includes all the apparatuses capable ofmaking effective use of STC. Typical apparatuses include an apparatusfor producing fumed silica by hydrolyzing the above STC with oxyhydrogenflames and an epitaxial apparatus for silicon wafers. Any knownapparatus may be used as the STC treating system.

As understood from the above description, according to the process ofthe present invention, the amount of the formed STC can be reduced to anextremely small value which is totally impossible with the Siemensprocess by carrying out the silicon deposition reaction between TCS andhydrogen at a high temperature range of 1,300° C. or higher. Thereby,the process of the present invention makes it possible to construct aclosed system that the formed STC is not discharged to the outside ofthe process.

In the silicon deposition reaction at the above high temperature range,by changing the ratio of hydrogen to TCS, the amount of the formed STCcan be adjusted to an extremely wide range without affecting the qualityof the obtained silicon. Thereby, even when a STC treating system isused, the amount of STC to be supplied to the system can be easilycontrolled and a well-balanced production mode can be employed.

EXAMPLES

The following examples are provided for the purpose of furtherillustrating the present invention but are in no way to be taken aslimiting.

Example 1

Silicon was produced in accordance with the process shown in FIG. 4 asfollows.

In the deposition step 101, the apparatus (deposition surface area ofabout 400 cm²) shown in FIG. 1 was used to supply TCS and hydrogen in ahydrogen/TCS molar ratio shown in Table 1 and heat the inner wall of thecylindrical vessel 1 at 1,450° C. so as to form silicon. TCS was mixedwith part of hydrogen and supplied from the chlorosilane feed pipe 4 andthe remaining hydrogen gas was supplied from the seal gas feed pipe 6 asa seal gas to ensure that the total amount of hydrogen should be asshown in Table 1. A small amount of hydrogen was supplied from the sealgas feed pipe 11 as a seal gas. The reaction pressure was 50 kPaG.

Table 1 shows the amount of the deposited silicon, the amount of theformed STC and the amount of the formed hydrogen chloride in the silicondeposition step. The amounts of the formed STC and hydrogen chloridewere calculated by analyzing the exhausted gas from the silicondeposition reaction by gas chromatography.

The exhausted gas in the above silicon deposition step was pressurizedat about 700 kPaG by a pressure device and heated to be supplied to theTCS forming step 102. In the TCS forming step, a fluidized bed reactorfor metallurgical-grade silicon as raw material silicon was used. Thereaction conditions of the TCS forming step included a temperature of350° C., a pressure of 700 kPaG and a raw material silicon charge ofabout 10 kg. The raw material silicon had an average particle diameterof about 200 μm and a purity of 98% and contained metals such as iron,aluminum, titanium and calcium as the main impurities and also carbon,phosphorus and boron.

A dried hydrogen chloride gas was supplied to the TCS forming step 102.The supply of the hydrogen chloride gas was used to maintain the contentof hydrogen in the system shown in Table 1. This amount was balancedwith chlorine contained in the chlorosilanes to be discharged to theoutside of the system such as the heavy end extracted by a distillationpurifying system and STC extracted as a surplus according tocircumstances.

In the TCS forming step 102, hydrogen chloride formed in the silicondeposition step 101 and the STC reducing step 104 to be describedhereinafter and hydrogen chloride supplied to maintain the content ofchlorine in the system were reacted with the raw material silicon toform TCS as the main product and STC as a by-product.

After fine powders of the raw material silicon accompanied by the gasdischarged from the TCS forming step 102 were removed by a filter, thegas was supplied to the hydrogen/chlorosilane separation step 201 to becooled to −30° C. in order to condensate parts of chlorosilanes, therebyseparating a hydrogen gas. The separated hydrogen gas was supplied tothe above silicon deposition step 101 and the STC reducing step 104 tobe described hereinafter.

When the hydrogen gas from which parts of chlorosilanes had beenseparated was let pass through a vessel filled with 10 liters of an ionexchange resin having a substituent —N(CH₃)₂ and fully dried, the purityof the silicon deposit was 50 Ω·cm for P type. When the ion exchangeresin was not used, the purity of the silicon deposit was 1 Ω·cm for Ptype.

Meanwhile, the condensed chlorosilanes were supplied to the distillationcolumn of the TCS first recycling step 103 to separate into TCS and theheavy end containing STC and heavy metals.

TCS and STC separated and purified in the TCS first recycling step 103were gasified, TCS was supplied to the silicon deposition step in anamount shown in Table 1, and STC was supplied to the STC reducing stepin an amount shown in Table 2. In the STC reducing step 104, a reductionreaction was carried out by setting the hydrogen/STC molar ratio to 10.As for the supply of hydrogen, the upper limit of the total amount ofhydrogen supplied to the silicon deposition step 101 and the STCreducing step 104 was about 50 Nm³/H according to the limitation of thepressure device. A similar reactor to the reactor shown in FIG. 1 wasused as the reactor of the STC reduction reaction to supply a mixed gasof STC and hydrogen from the chlorosilane feed pipe 4. Other operationconditions were the same as the silicon deposition reaction.

Table 2 shows the reaction conditions of the STC reduction reaction andthe amount of TCS formed by the reaction. The amount of the formed TCSwas calculated by analyzing the exhausted gas from the STC reductionreaction by gas chromatography.

The gas exhausted from the STC reduction step was supplied from the TCSsecond recycling step 105 to the above TCS forming step 102.

Table 3 shows the amount of surplus STC, the amount of surplus STC basedon 1 kg of the produced silicon, the total supply of hydrogen for thedeposition reaction and STC reduction reaction and the supply ofhydrogen based on 1 kg of the produced silicon for evaluating the effectand economic efficiency of the above closed system.

The surplus STC refers to STC which must be discharged to the outside ofthe system in the STC extraction step 106 as shown in FIG. 5 because alarger amount of STC is still formed by the silicon deposition reactionor the like even when the maximum amount of STC which can be supplied tothe STC reduction reaction is supplied.

As described above, according to the present invention, it is understoodthat the formation of STC can be suppressed in the silicon depositionstep 101 and that a closed system for preventing the formation ofsurplus STC can be constructed even with a small-sized STC reductionreactor.

Examples 2 and 3

Silicon was produced in the same manner as in Example 1 except that themolar ratio of hydrogen to TCS was changed as shown in Table 1 in thesilicon deposition step 101.

The amounts of the formed products in each step are shown in Tables 1 to3 in the same manner as in Example 1. It is understood that aneconomical closed system can be constructed like Example 1.

Example 4

15.6 kg/H of surplus STC could be formed as shown in Table 3 by changingthe molar ratio of hydrogen to TCS in Example 1 to 5 in the silicondeposition step 101. This was calculated to be 14.9 kg of surplus STCbased on 1 kg of the produced silicon. Thus, the same amount of STC assurplus STC obtained by the Siemens process to be described hereinaftercould be obtained.

Examples 5 and 6

The procedure of Example 1 was repeated except that the depositionreaction temperature in the silicon deposition step 101 was changed to1,350° C., the molar ratio of hydrogen to TCS was changed as shown inTable 1 and the reduction reaction temperature of the STC reducing step104 was changed to 1,350° C. as shown in Table 2. The temperature of theinner wall of the cylindrical vessel was raised to 1,450° C. or higherfor 5 minutes once every hour to continue the reaction while the depositwas molten and dropped intermittently.

As a result, a closed system could be constructed like Example 1 asshown in Table 3.

As shown in the above Examples 1 to 6, according to the presentinvention, it is understood that an extremely wide range of operationfrom the construction of a perfect closed system to the acquisition of alarge amount of surplus STC is possible with apparatuses of the samescale by setting the deposition reaction temperature at 1,300° C. orhigher in the silicon deposition step 101 and changing the molar ratioof hydrogen to TCS. That is, according to the present invention, it ispossible to cope with a change in demand from an STC treating systemflexibly without changing the scale of the silicon production apparatus.

Comparative Example 1

A bell-jar type reactor (deposition surface area of about 1,200 cm²)which is generally used in the Siemens process was used in the silicondeposition step 101 and the STC reducing step 104 of Example 1. Thedeposition reaction temperature and the reduction reaction temperaturewere set to 1,150° C. which was the upper limit temperature able to beset industrially in the Siemens process of the prior art, and thereaction pressure was 50 kPaG like Example 1.

The molar ratio of hydrogen to TCS in the silicon deposition step 101was 10 because the industrial upper limit for smoothening the shape ofthe silicon deposit and maintaining a stable deposition reaction wasabout 10.

The other steps were carried out under the same conditions as in Example1.

It is understood from the above Comparative Example that when thedeposition of silicon is carried out at a temperature equal to or lowerthan the melting point of silicon, particularly by the Siemens processof the prior art, a large amount of surplus STC is formed.

As understood from comparison between Comparative Example 1 and Example4, the output of silicon in Example 4 is about 4 times larger than inComparative Example 1 even when the apparatuses of the same scale areused, and the process of the present invention has extremely excellenteconomic efficiency.

TABLE 1 Amount of hydrogen Amount of Reaction for Supply of depositedAmount of Amount of Supply of temperature reaction TCS Hydrogen/TCSsilicon formed STC formed HCl HCl ° C. Nm³/H kg/H molar ratio kg/H kg/Hkg/H kg/H Ex. 1 1,450 30 12 15 0.51 4.6 1.3 0.30 Ex. 2 1,450 30 9.1 200.50 2.5 1.8 0.20 Ex. 3 1,450 30 6.0 30 0.37 1.5 1.3 0.14 Ex. 4 1,450 3018 5 1.05 17 1.1 14.1 Ex. 5 1,350 23 14 10 0.50 5.5 1.3 0.37 Ex. 6 1,35030 7.4 25 0.43 1.8 1.6 0.18 C. Ex. 1 1,150 30 18 10 0.27 5.3 0.05 4.0

TABLE 2 Amount of hydrogen Reaction for Supply of Amount of temperaturereaction STC formed TCS ° C. Nm³/H kg/H kg/H Ex. 1 1,450 20 15.8 4.8 Ex.2 1,450 13 9.5 2.9 Ex. 3 1,450 8 5.9 1.8 Ex. 4 1,450 20 15.8 4.8 Ex. 51,350 26 19.5 5.6 Ex. 6 1,350 10 7.5 2.2 C. Ex. 1 1,150 20 15.8 2.8 Ex.= Example, C. Ex. = Comparative Example

TABLE 3 Amount of Total amount of hydrogen Total amount of hydrogensurplus STC recycled for deposition recycled based Amount of based on 1kg of reaction and STC on 1 kg of surplus STC produced silicon reductionreaction produced silicon kg/H kg Nm³/H Nm³/H Ex. 1 0 0 51 100 Ex. 2 0 043 86 Ex. 3 0 0 38 103 Ex. 4 15.6 14.9 51 49 Ex. 5 0 0 48 96 Ex. 6 0 041 94 C. Ex. 1 4.0 14.9 51 189 Ex. = Example, C. Ex. = ComparativeExample

1. A silicon production process comprising: a silicon deposition stepfor forming silicon by reacting trichlorosilane with hydrogen at atemperature of 1,3000° C. or higher; a trichlorosilane forming step forforming trichlorosilane by contacting the exhausted gas in the silicondeposition step to raw material silicon to react hydrogen chloridecontained in said exhausted gas with silicon; and a trichlorosilanefirst recycling step for separating trichlorosilane from the exhaustedgas in the trichlorosilane forming step and recycling it to the silicondeposition step.
 2. The process of claim 1, wherein the molar ratio ofhydrogen to trichlorosilane is 10 or more in the silicon depositionstep.
 3. A silicon production process comprising: a silicon depositionstep for forming silicon by reacting trichlorosilane with hydrogen in ahydrogen/trichlorosilane molar ratio of 10 or more at a temperature of1,3000° C. or higher; a trichlorosilane forming step for formingtrichlorosilane by contacting the exhausted gas in the silicondeposition step to raw material silicon to react hydrogen chloridecontained in said exhausted gas with silicon; a trichlorosilane firstrecycling step for separating trichlorosilane from the exhausted gas inthe trichlorosilane forming step and recycling it to the silicondeposition step; a tetrachlorosilane reducing step for obtainingtrichlorosilane by reducing tetrachlorosilane contained in the residueafter the separation of trichlorosilane in the trichlorosilane firstrecycling step with hydrogen; and a trichlorosilane second recyclingstep for recycling the exhausted gas in the tetrachlorosilane reducingstep to the trichlorosilane forming step.
 4. A silicon productionprocess comprising: a silicon deposition step for forming silicon byreacting trichlorosilane with hydrogen at a temperature of 1,3000° C. orhigher; a trichlorosilane forming step for forming trichlorosilane bycontacting the exhausted gas in the silicon deposition step to rawmaterial silicon to react hydrogen chloride contained in said exhaustedgas with silicon; a trichlorosilane first recycling step for separatingtrichlorosilane from the exhausted gas in the trichlorosilane formingstep and recycling it to the silicon deposition step; atetrachlorosilane reducing step for obtaining trichlorosilane byreducing part of tetrachlorosilane contained in the residue after theseparation of trichlorosilane in the trichlorosilane first recyclingstep with hydrogen; and a trichlorosilane second recycling step forrecycling the exhausted gas in the tetrachlorosilane reducing step tothe trichlorosilane forming step; and a tetrachlorosilane supply stepfor supplying the balance of tetrachlorosilane supplied to thetetrachlorosilane reducing step to a tetrachlorosilane treating system,wherein the amount of tetrachlorosilane to be supplied to thetetrachlorosilane treating system is changed in the tetrachlorosilanesupply step by changing the molar ratio of hydrogen to trichlorosilaneto be supplied to the silicon deposition step.
 5. The process of claim 3or 4, wherein the reaction temperature in the tetrachlorosilane reducingstep is 1,3000° C. or higher.
 6. The process of any one of claims 1, 3and 4, wherein the exhausted gas which is contacted with the rawmaterial silicon in the trichlorosilane forming step contains hydrogenchloride and tetrachlorosilane as main components.